Resistance to Shape Refinement of Precipitates in Al-(Si-Ge) Alloys during Thermal Cycling.

V. RADMILOVIC; D. MITLIN; A. TOLLEY; U. DAHMEN; J. W. MORRIS JR.

METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE

SPRINGER

Año: 2003 vol. 34 p. 543 - 551

1073-5623

The present work addresses the response of Si-Ge precipitates in Al-0.5Si-0.5Ge (at. pct) to thermalcyclingtreatments of the sort known to refine the shapes of Ge precipitates in binary Al-Ge alloys.Alloys aged at 250 C contained both small, platelet precipitates on {111} planes and larger, equiaxedprecipitates that were heavily twinned. Thermal cycling between 250 C and 360 C led to partial orcomplete dissolution of the platelets. However, the equiaxed particles coarsened at an essentiallyconstant shape; the shape refinement that led to untwinned, single-variant octahedral precipitates inbinary Al-Ge did not occur. The apparent reason is the heavy twinning of the Si-Ge precipitates,which produces particles with a nearly spherical shape and rounded, incoherent interfaces and is,hence, a viable mechanism for relaxing the large misfit strain of the precipitate structure. The twinnedparticles undergo normal coarsening at an essentially constant shape. After thermal cycling, theprecipitates contain Si and Ge in the approximate ratio of 70Si-30Ge, which is in the compositionrange expected for the cycling temperature.